Auditory Effects of Blast Exposure

March 2014

Jeanne Dodd-Murphy, PhD, CCC-A

Community Outreach Following an Industrial Explosion

Commemorations in April 2014 of the 1-year anniversaries of the Boston Marathon bombing and the fertilizer company explosion in West, Texas, have once again raised public interest in blast-related injuries and their lingering effects on individuals and communities. In the aftermath of such devastating events, those injuries that are potentially life-threatening or immediately debilitating are given the most attention. Auditory injuries are among the most common primary injuries resulting from blast exposure; however, hearing loss may be overlooked in the midst of more urgent concerns (Centers for Disease Control and Prevention [CDC], 2003; Myers, Wilmington, Gallun, Henry, & Fausti, 2009).

This article briefly reviews blast injuries to the auditory system and describes clinical insights gained by faculty and students from Baylor University while they were providing hearing health services in the wake of the industrial explosion in West.

On the night of April 17, 2013, a fire at a fertilizer business in the town of West, Texas, ignited a large supply of ammonium nitrate, causing an explosion that left a crater 93 feet wide and 10 feet deep, destroyed a nearby nursing home and apartment complex, and caused irreparable damage to West's intermediate and high schools. The massive blast resulted in the destruction of 150 homes and damaged 200 more, breaking windows seven miles away in Abbott, Texas. Fifteen people were killed, including 12 first responders who were in and around the structure when it exploded; conservative estimates placed the number of injured at over 200 (Ambrose, 2014; Swanson & Tarrant, 2013). The small town of West (population, approximately 2,800) is Waco's neighbor to the north—about 20 miles from Baylor's main campus. Faculty and students from Baylor's Communication Sciences & Disorders Department were among the multitude of volunteers who offered assistance afterward and just one group out of numerous health care providers volunteering to donate hearing-related services to citizens of West who were affected by the blast.

Audiologists working outside of Veterans Affairs (VA) settings in the United States rarely have extensive experience with blast-exposed patients. In preparation for working with individuals from West, Baylor audiology faculty reviewed pertinent literature for blast-related consequences, particularly factors that might increase the risk of long-term hearing loss.

The category and amount of explosive affect the type and distribution of resulting injuries. Ammonium nitrate, stored by the West Fertilizer Company, is classified as a high-order explosive; this category of explosives is characterized by the presence of an immense positive-pressure "blast wave" upon detonation (CDC, 2003). The chemical reaction initiated by the fire likely detonated the fertilizer pellets, almost instantaneously converting them to a gaseous form and releasing energy estimated to be at least 3 times greater than that of the fertilizer bomb used in the Oklahoma City bombing in 1995 that destroyed the Murrah Federal Building (Swanson & Tarrant, 2013). The brief blast wave travels faster than the speed of sound, radiating in all directions from the explosion site. As its distance increases from the epicenter, the blast wave loses energy and slows down, becoming an acoustic wave. Behind the blast wave is a slightly longer negative pressure wave; then the final phase is what is known as the "blast wind." A higher amount of detonated explosive increases the distance that the blast wave travels from the epicenter and the damage done by the sound wave beyond that radius, presumably increasing the risk of permanent hearing loss.

If the explosion occurs within a contained space, reflections of the blast wave can increase the impact (CDC, 2003). In West, the fertilizer was in a contained space, but the blast wave disintegrated the buildings and traveled outward, causing complete destruction of some homes over half a mile away, so people within about a half mile radius of the blast would likely be the most affected by the blast wave and the most likely to incur primary blast injuries. Numerous other factors—such as the presence of any barriers and whether the individual was inside a building, outdoors, or in a vehicle and even the orientation of a person's body and head relative to the explosion site—would determine the presence, nature, and extent of any injuries. The variability of outcomes for survivors of blast exposure is related to a complex pattern of circumstances and often difficult to predict (CDC, 2013; Myers et al., 2009; Van Campen, Dennis, Hanlin, King, & Velderman, 1999).

Regardless of how erratic their patterns seem, blast injuries are typically classified as primary, secondary, tertiary, or quaternary. Primary injuries result from barotrauma from the initial blast wave impacting the body. Air-filled cavities—particularly the lungs, middle ear, and intestines—are the most susceptible to damage. Tympanic membrane (TM) perforation is typically the most common primary blast injury, because the TM will rupture at lower pressures than the lungs or gastrointestinal organs. The presence of TM perforation is considered a physical marker of significant blast exposure, but the absence of TM perforation(s) does not preclude the existence of serious blast injury. Another common primary blast injury is concussion; no visible signs of head injury may be present in concussion, but the extreme positive-pressure wave may cause the brain to move inside the skull, resulting in traumatic brain injury (TBI) ranging from mild to severe. Secondary injuries include penetrating wounds or blunt trauma caused by flying debris, while tertiary injuries result from the body being thrown by force against another object. Finally, quaternary blast injuries are due to any other mechanisms; these types of injuries would include burns, crush injuries, and pulmonary disease from inhalation of dust, smoke, or chemicals (CDC, 2003; Myers et al., 2009).

Blast exposure effects on the auditory system can include peripheral hearing impairment, tinnitus, auditory processing disorder, and vestibular impairment. The blast wave, or a secondary or tertiary mechanism, may result in ossicular separation in addition to TM perforation, resulting in conductive hearing loss. Epithelial cells from the TM may be propelled into the middle ear space in the case of TM perforation, increasing the risk of choleasteatoma. Though an immediate TM perforation may reduce low- to mid-frequency cochlear loss, the blast wave can also damage the cochlea; additionally, the subsequent intense acoustic wave can result in cochlear damage, affecting individuals at a greater distance from the explosion site. The blast wave can injure the inner ear without damaging the middle ear, potentially leaving permanent sensorineural hearing loss and/or tinnitus. Cochlear damage incurred from an extreme pressure wave is more likely to result from mechanical damage—such as a perilymph fistula, basilar membrane rupture, and damage or destruction of stereocilia and/or hair cells—than from metabolic changes that have been observed with occupational noise exposure over time. TBI due to primary, secondary, tertiary, and/or quaternary injuries is associated with an increased risk of hearing loss. Even closed head injury may affect the cochlea and/or the central auditory pathways as well as the conductive system. During emergency care, sensorineural hearing loss is not a main focus of treatment, but can complicate assessment, giving the impression of nonresponsiveness or cognitive deficit. Delayed onset of TBI symptoms may occur after the initial medical treatment, so medical monitoring is important for patients with blast exposure. Individuals who report hearing loss following an explosion may experience some short-term recovery of temporary threshold shifts over hours, days, or weeks afterward; thus, they may not seek follow-up on their remaining hearing impairment, or they may delay having an evaluation (Myers et al., 2009).

The majority of reports of large-scale acoustic trauma and other auditory injuries related to blast exposure have focused on military personnel, emphasizing immediate casualties and the emergency care of otologic injuries. One exception is the work of Van Campen and colleagues (Van Campen et al.,1999), who evaluated 83 survivors of the 1995 Oklahoma City bombing quarterly across the year after the blast, tracking the onset, persistence, and severity of ear and hearing-related symptoms along with data from audiologic evaluations. A thorough questionnaire specifically designed for the research established what ear-related symptoms were experienced after the blast and whether those symptoms remained at 1 year post blast. The most frequently reported symptoms occurring shortly after the blast were tinnitus and distorted hearing; two thirds of those with blast exposure indicated these auditory symptoms. Other immediate symptoms reported were loudness sensitivity, ear pain, and dizziness/vertigo. Though there was slight improvement of ear-related symptoms over the first 6 months, on average, the proportion of reported tinnitus, distorted hearing, loudness sensitivity, and ear pain was unchanged after 1 year. Averaged across the year, 76% of the participants with blast exposure had hearing loss at one or more frequencies. Most cases showed sensorineural hearing loss, and bilateral loss was present in 74% of the group. Though varying hearing loss patterns were present, the most frequent configuration was sloping rather than the notched configuration that is often observed with hearing loss induced by occupational or recreational noise. These results are consistent with other reports of hearing loss related to blast injury: bilateral symmetrical sloping high frequency mild to moderate sensorineural hearing loss is most common, but a diverse range of configurations and degrees are possible.

Faculty and students from Baylor's Communication Sciences and Disorders Department participated in a health fair at the West Community Center in September 2013, 5 months after the explosion. The health fair had been planned by West's long-term recovery task force, who requested that Baylor audiology faculty assist by providing hearing screenings and education about hearing loss. The event was centered around hearing and audiology services in response to widespread concern from West citizens about lingering hearing problems related to the blast.

A team of two Baylor audiology faculty and numerous students with CSD majors participated in the health fair. A large audiology booth included a table display, give-away items (e.g., foam ear plugs, bookmarks with dB level charts), and copies of educational materials related to hearing assessment and hearing loss, noise and hearing conservation, and assistive technology. One audiologist was available at the booth at all times to answer questions and supervise the students who assisted in providing hearing health education. At least 40 individuals visited the audiology booth with questions, but chose not to have a hearing screening at that time. Our experiences in West reinforced the importance of counseling in audiological practice, particularly after such a traumatic event. Students observed later that many people who came to the screening or the hearing information booth just wanted to share their stories. Listening with interest and acknowledging the feelings of each individual are as much a part of the therapeutic process as distributing educational material or answering questions about hearing loss.

Baylor University CSD faculty and students pose with Tommy Muska, mayor of West, at a community health fair in September 2013.

The pure-tone screenings were conducted in a separate quiet room. An audiologist was present at all times to supervise and assist CSD master's students who conducted the screenings. Prior to testing, health fair attendees completed a case history questionnaire. The need for case history information specific to blast exposure during clinical assessment was emphasized in the literature. The questionnaire used by Van Campen et al. (1999) was adapted as a one-page history form for screening purposes. Under the circumstances, the persistence of ear-related symptoms, such as tinnitus, ear pain or pressure, and/or dizziness, was considered sufficient to warrant otologic referral, regardless of the pure-tone screening outcome. Pure-tone screening was administered at 25 dB HL at 1000, 2000, and 4000 Hz bilaterally; missing one or more tones in either ear resulted in referral for a complete audiologic evaluation. Some results are described below for adults who completed both the questionnaire and pure-tone screening.

Of 39 people who had pure-tone screenings and completed the questionnaires, 10 were not within hearing distance of the explosion; 90% of this group passed the pure-tone screening. In contrast, only 24% of participants reportedly within hearing distance of the blast passed the pure-tone screening. Most of the participants who had been exposed to the blast were less than one mile from the explosion site, with a smaller proportion located between a mile and 1.5 miles away. Only four individuals who were screened noted receiving treatment after the explosion; so, the majority of the blast-exposed screening participants who may have hearing loss were not likely to have been included in the official count of the injured—which count focused on medical records from hospitals known to have treated survivors. According to Van Campen et al. (1999), hearing impairment and related symptoms persisting at 6 months post blast are likely to be permanent. Because the health fair occurred 5 months after the West explosion, those who failed to pass the screening (76% of the blast-exposed group) are considered to be at risk for permanent hearing impairment.

Statements regarding type, degree, and configuration of hearing loss cannot be made based on results from a pure-tone screening; however, most of those who were referred missed at least one tone in each ear, indicating potential bilateral loss. Pure tones at 4000 Hz were missed most frequently; only two of the participants who were referred detected 4000 Hz in each ear but missed some combination of the other two frequencies. Tones at 2000 Hz were missed more often than those at 1000 Hz, suggesting that higher frequencies may have been more affected.

Subjective ear-related symptoms noted on the case history were consistent with the findings of Van Campen et al. (1999). For the "refer" group (those who missed at least one screening tone in either ear), tinnitus was the most frequently reported immediate symptom after the blast. Two people indicated that the tinnitus had resolved by the time of the screening. All of those who reported having muffled/distorted hearing and sensitivity to loud sounds shortly after the explosion indicated that they were still experiencing those symptoms at the time of the screening. A smaller number of people reported ear pressure (some indicated this symptom was no longer present) and dizziness, including two participants who passed the pure tone screening. Only a few participants had had complete hearing tests after the explosion. Though a number of participants indicated ear pain following the explosion, none of them reported persistent ear pain at 5 months post blast, while the majority of survivors of the Oklahoma City bombing continued to report ear pain at 1 year following the explosion (Van Campen et al., 1999). Just over half of the individuals who failed to pass the pure-tone screening reported persistent ear-related symptoms that began immediately after the explosion. This reporting likely underestimates the incidence of ear-related symptoms at the time of the screening: there were missing data from some case histories; completing history forms as part of follow-up interviews may have rendered more accurate information. Most of those noting symptoms reported bilateral symptoms; this was true even for participants who failed the screening based on 4000 Hz unilaterally. In addition, half of the six blast-exposed participants who passed the screening reported persistent symptoms despite being able to detect the tones at the screening level.

There is a need for audiological outreach and education for survivors of an explosion; hearing loss that does not require immediate medical treatment after blast exposure may go unevaluated indefinitely. In West, those individuals who lived nearest the explosion site were more likely to be affected by the blast physically, but were also more likely to have suffered property loss or damage. In the midst of grief and displacement, seeking services for hearing was considered a low priority. Some were not aware of the nature of audiology services or perceived that "nothing could be done" about hearing loss. Numerous individuals did not consider themselves injured, even though they experienced auditory symptoms, possibly because they thought of tinnitus and hearing loss as natural consequences of the blast. There also seemed to be a tendency for many individuals from West to downplay their own issues because of a prevailing attitude that others had lost so much more. Nevertheless, once West citizens could focus on long-term recovery, hearing issues were at the forefront of health concerns. Hearing health services are likely to be maximally effective when they address needs recognized by the community itself and at a time when members of the community are ready to take action.

About the Author

Jeanne Dodd-Murphy, PhD, CCC-A (Jeanne_Murphy@baylor.edu), is an audiologist and associate professor in the Department of Communication Sciences & Disorders at Baylor University. Her research interests include the efficacy of hearing screening in school settings and the use of functional outcomes measures to determine potential effects of hearing loss on daily communication and socio-emotional function for individuals of all ages. Dr. Dodd-Murphy has over 25 years of experience as a clinical and/or educational audiologist.

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